The invention relates to new and useful improvements in circuit arrangements for accurately detecting a direct current derived from clocked electric input values.
Current detection is normally carried out by first converting the current to a proportional voltage, prior to further processing. The traditional method for further evaluation is to evaluate the voltage drop caused by the current to be measured across a measurement resistor. However, as the current levels rise, this results in problems. Depending on the size of the measurement resistor used, this either leads to high power losses, or smaller measurement voltages must be used in order to limit the power loss. This, however, reduces the measurement accuracy.
In one special case, the direct current to be measured is derived from clocked electrical input variables. FIG. 1 shows the block diagram of a circuit suitable for this purpose, specifically a basic circuit of a so-called pulse controller PS which includes a chopper Z provided with an input DC voltage Ug. The chopper Z uses this to produce a pulsed intermediate voltage Up and a pulsed intermediate current Ip which has, for example, a trapezoidal waveform. The chopper Z is followed by a smoothing device GA for the intermediate voltage Up and for the intermediate current Ip, and this converts the pulsed variables to an output DC voltage Ua and an output direct current Ia. The chopper Z receives a predetermined nominal value Ua* for the actually desired magnitude of the output DC voltage Ua. This is essentially used to influence the frequency or pulse width of the intermediate voltage Up and the intermediate current Ip such that the intermediate voltage Up assumes a desired value.
FIG. 2 shows, inter alia, the basic circuit of a smoothing device GA in the pulse controller PS. This smoothing device GA has a smoothing inductor L with a freewheeling diode Vf, to which the pulsed intermediate voltage Up and the pulsed intermediate current Ip are supplied. The majority of the alternating component of this current is absorbed by a first smoothing capacitor Ca, so that a virtually pure DC voltage Ua is produced at the output of the circuit in FIG. 1, and the output direct current Ia corresponds to the mean current Il through the smoothing inductor L.
FIGS. 3A, 3B and 3C show the waveforms of the most important electrical signals in the circuit in FIG. 2. For example, FIG. 3A shows three pulses P1, P2, P3 from the waveform of the pulsed intermediate voltage Up at the circuit input. FIG. 3B shows the waveforms of the triangular current Il through the smoothing inductor, and of the output direct current Ia. The current Il through the smoothing inductor L is composed of two components. For the duration of each of the pulses P1, P2 and P3, the current Il corresponds to the waveform of the pulsed intermediate current Ip while, in the interims between the pulses, it corresponds to the trapezoidal current If through the freewheeling diode Vf.
The circuit in FIG. 2 also includes a component M1 which can be used to detect the present magnitude of the output direct current Ia, in a known manner. This first current measurement circuit M1 includes a first measurement resistor Rs1 through which the current passing through the smoothing inductor L flows and across which a first measurement DC voltage Um1 can be detected, as a measure of the magnitude of the output direct current Ia. The already mentioned drawback of considerable power losses in the measurement resistor Rs1 occurs with this circuit.
A second current measurement circuit M2, which is also illustrated in the circuit example in FIG. 2, uses a current transformer T to avoid virtually all the power losses in the measurement resistor. The transformation ratio of this current transformer T can be chosen to achieve a sufficiently large pulsed measurement current Im for virtually any given value of the pulsed intermediate current Ip. As a first component element, the current measurement circuit M2 includes a circuit Ms which is used to detect the pulsed intermediate current Ip and to convert it, as a floating potential, into a pulsed measurement current Im on the secondary of the current transformer T. The pulsed measurement current Im is passed to a first rectifier diode Vs and a second measurement resistor Rs2. The pulsed measurement voltage Us appears across this measurement resistor Rs2, which effectively forms the output of the current detection circuit Ms.
Since direct current cannot pass through the current transformer T, the measurement current Im for the second measurement resistor Rs2 is available only in pulsed form. To obtain a continuous measurement signal once again, a so-called peak-value detector Sg is provided downstream of the current transformer T. The circuit in FIG. 2 shows one example of such a peak-value detector Sg, comprising a second rectifier diode Vg and a parallel circuit formed by a damping resistor Rm and a second smoothing capacitor Cm. A second measurement DC voltage Um2 is thus available at the output of the second measurement circuit M2.
Even though this peak-value detector Sg produces a continuous measurement DC voltage Um2 from the pulsed measurement current Im, there is nonetheless an associated drawback. Specifically, this measurement DC voltage Um2 is only approximately proportional to the desired mean value of the current Ip or to the desired value of Ia. The accuracy of this measurement circuit M2 is also adversely affected by temperature-induced variations in the forward voltage of the second rectifier diode Vg. This will be explained in more detail in the following text with reference to the signal waveforms shown in FIG. 3C.
FIG. 3C shows the pulses of the pulsed measurement current Im which flows through the secondary of the current transformer T. This produces a pulsed measurement voltage Us across the second measurement resistor Rs2 at the output of the current detection circuit Ms. This is described by the relationship Us=Rs2*Im. An ideal peak-value detector would convert the pulsed measurement voltage Us into the measurement DC voltage Um*, whose magnitude would correspond to the peak value of the pulsed measurement voltage Us. However, in fact, owing to the non-ideal characteristics of the rectifier diode Vg in particular, a measurement DC voltage value Um2 is actually produced at the output of the peak-value detector Sg in FIG. 2.